Liquid crystal clinical thermometer

ABSTRACT

A method is disclosed for improving the signal retention of a liquid crystal sensing element comprising an embossed cavity containing the liquid crystal and a cover layer sealing the liquid crystal in the cavity by heat treating at about 95° to about 120° C. for a time sufficient to cause the embossed cavity to shrink to substantially its original form thereby forcing the liquid crystal to be forced up against the cover layer to form a domed structure. Essential to the success of the process is incorporation in the cavity of about 10 to about 50% by volume of air based on the volume of the cavity.

FIELD OF THE INVENTION

This invention relates to the field of clinical thermometers. Inparticular it relates to an improved liquid crystal clinicalthermometer.

BACKGROUND OF THE INVENTION

The thermochromic property of cholesteric liquid crystal compounds hasinvited considerable effort for their application to thermometerinventions. Many of the liquid crystal thermometers described in priorart are alleged for the measurement of human body temperature; however,none has been a commercial success because they all fail in one or moreof their attributes to satisfy the needs of the medical community.

The liquid crystal forehead thermometer is a good example of one suchproduct. Because of its low resolution, +1° C. it has principally servedas a screening device for fever, requiring subsequent confirmation oftrue body temperature with a mercury-in-glass or electronic clinicalthermometer. This is not, however, its only serious drawback. Theforehead has been found to be a unreliable site for representing coretemperature, failing more than 50% of the time to detect fever (falsenegatives).

A medically acceptable clinical thermometer based on liquid crystalcolor changes must meet the same exacting standards for range (35°-41°C.), resolution (0.1° C.), accuracy (0.1° C. in the critical range, 0.2°C. elsewhere), and stability as do the mercury in glass and electronicclinical thermometers. Furthermore, it must be (i) designed to measurethe temperature at a site which is accepted in the medical community asfaithfully representing core temperature; e.g., rectum, sublingualcavity or axilla, (ii) easy to read, clean and reset between uses, (iii)safe and comfortable when used at the site, (iv) and retain its accuracyfor at least five years during storage and distribution when subjectedto temperature extremes of -20° to 60° C. It should also exhibit somespecial features which result in benefits not shared by competitiveproducts; e.g., less expensive, easier to use, unbreakable, no powerrequirement, child friendly, etc.

U.S. Pat. No. 3,974,317, incorporated herein by reference, describes athermometric composition which fulfills the requirements regardingrange, resolution, accuracy and stability. This patent describes acholesteric liquid crystal system which can be used to constructthermometric elements capable of recording numerous increments intemperature from a single basic composition in a facile and economicmanner.

Each thermometric element of this invention comprises a plurality ofseparate compositions having identical colors when viewed on an inertblack background. Each distinct composition is capable of recordingtemperature by a visual change in color and is comprised of a mixturecontaining:

a. a first cholesteric liquid crystal system characterized by exhibitingcolor in the cholesteric state at a first temperature and changing fromthis state to a second state exhibiting a different color at a secondtemperature, and

b. a second component other than (a) which is a chemically inertsubstance miscible with (a); the same second component in differingpercentages by weight thereof, being utilized in each compositionwherein said liquid crystal systems are identical, the differing amountsof said second component in each composition wherein the liquid crystalsystems are identical being in a predetermined weight range whereinthere is a predictable variability in a curve in which the temperatureat which said visual change in color takes place is plotted againstpercent by weight of said second component.

This '317 patent thus demonstrates that it is possible to change thephase transition temperature, referred to in the art as the clearingpoint, or focal conic state, of a liquid crystal system in a predictablemanner by varying the amount of the second component (b) in thecomposition.

An illustrative example of a composition taught in the '317 patent is aliquid crystal system comprising 57.9% cholesteryl oleyl carbonate 30.7%cholesteryl chloride and 11.4% cholesteryl-n-butoxyphenyl carbonate ascomponent (a). This composition has a clearing point at 54.0 C. Whenmineral oil, component (b), is added to (a), the clearing pointtemperature is depressed as a linear function of the percent by weightof the mineral oil added to (a). A plot of the clearing pointtemperature vs. weight percent mineral oil has a negative slope of 2.98°C. per one percent change in the mineral oil content of the composition,e.g., a composition containing 5.70 weight percent mineral oil wouldexhibit a clearing point at 37.0° C. whereas, a composition containing5.67 % weight mineral oil would exhibit a clearing point 0.1° higher or37.1° C.

Because the colors of all of the compositions containing both (a) and(b) are identical both below and above their respective clearing points,and because the change in color is not subjective, temperaturedifferences as small as 0.1° C. can be easily resolved, thus making thecompositions of U.S. Pat. No. 3,974,317 ideally suited for applicationin a clinical thermometer.

This chemistry, while necessary, is insufficient for making a clinicalthermometer. What also must be specified are the heat sealable substrateand transparent covering film that contain the liquid crystalcompositions and are inert relative to these compositions. Constraintson these materials are set forth in U.S. Pat. No. 4,064,872,incorporated herein by reference. Here it is taught that for thepreparation of thermometers useful for medical diagnosis, the separatefilms comprising the heat sealable sheet material and the carriersubstrate should contain less than 1 mg per square meter of componentswhich will react with the liquid crystals, either during manufacture orstorage. These potentially reactive materials may be residuals from themanufacturing process of the film such as monomers, solvents, inhibitorsor processing aids which may react with or dissolve in the liquidcrystal composition selected, resulting in modification of thetemperature at which color change will take place. The '872 patentteaches the use of polyvinyl chloride (PVC) and polyvinylidene chloride(PVDC) coated laminates as the heat sealing material. This patentemphasizes that the materials selected to enclose the liquid crystalcompositions should be of as low thermal mass as is possible consistentwith sufficient durability to allow for repeated use.

The '872 patent additionally describes a method for constructing aclinical thermometer from these compositions by arranging them in a dotmatrix array. Combining the teachings of this patent with thosedescribed in the '317 patent results in a clinical thermometer with therequisite precision, stability and accuracy set forth in the abovediscussion. Furthermore, this chemistry has been found to be nontoxicwhen tested on laboratory animals at doses where comparable levels of acommon toothpaste led to fatalities in all the animals tested. Plasticthermometers made with this chemistry are unbreakable and can be usedhundreds of times without loss of efficacy.

Thermometers made using the combined teachings of '317 and '872 patentssuffer from a serious technical shortcoming relating to readability.Because of the small size of the individual dots, 1 mm diameter, the lowcontrast between the green liquid state and the gray focal conic state,and the relatively short duration of the signal, 15-20 seconds, beforereversion begins, those unfamiliar with reading the thermometer or thoseattempting to read it in lighting of low intensity will experiencedifficulty. Because of this deficiency, clinical thermometers made usingthese teachings have experienced limited commercial success.

When most liquid crystal thermometers are removed from one environmentin their range of transition to a lower temperature, the signal fades sorapidly that it is not possible to obtain an accurate temperaturedetermination of the first environment. This is for two reasons.Firstly, like all thermometers, those made of liquid crystals are of lowthermal mass and cool quickly. Secondly, nearly all liquid crystalcompositions respond with time constants of less than one second,whether it is exposed to a temperature increase or decrease and thusdisplay exceedingly short memory.

The chemistry taught in '317 does not differ from other liquid crystalthermometer chemistries in this respect for its transition from itsclear isotropic state to a cloudy focal conic state is essentiallyinstantaneous. For reasons which are not well understood, however, theparticular combination of liquid crystals described in the '317 patentbecomes trapped briefly in this focal conic state undergoing a rathersluggish transition from this state to the brightly colored liquidcrystal state. This brief memory is quite sensitive to the temperaturedifference between the test and reading environments. For example, aliquid crystal thermometer removed from a 37° C. mouth and returned to aroom temperature of about 20° C. would retain a high contrast signal forapproximately 20 seconds before disappearing; whereas, the samethermometer withdrawn from a mouth at 40° C. and returned to roomtemperature would retain its signal for only 10 seconds.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an individual pocket of the prior art thermometer filledwith liquid crystal chemical;

FIG. 2 shows a typical Centigrade Scale thermometer;

FIG. 3 shows a typical Fahrenheit Scale thermometer;

FIG. 4 shows the inverted pocket of this invention;

FIG. 5A shows a partial cut away elevation view of a heating unit forproducing the cavity inversion of this invention;

FIG. 5B is a cross sectional view along section line A--A as shown inFIG. 5A.

FIGS. 6A and 6B show a typical clinical thermometer for use on theabdomen;

FIG. 7 shows an elevation view of sealing equipment; and

FIG. 8 is an elevation end view of the equipment shown in FIG. 7.

SUMMARY OF INVENTION

It has surprisingly been found that by containing the liquid crystalcomposition in a pocket or cavity having a domed, inverted structure andan air void comprising about 5 to about 50% of the volume of the cavity,results in a thermometer which overcomes the deficiencies of the priorart liquid crystal devices in that the signal is retained indefinitely,while at the same time being fully reversible by the application ofpressure to the liquid crystal containing region of the thermometer.Concomitantly, this modification also enhances the contrast between thetwo states, liquid crystal and focal conic, useful for temperaturemeasurement.

The inversion of the thermometer pocket is accomplished by subjectingthe thermometer of conventional structure to elevated temperatures.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to a structural modification in the prior artliquid crystal thermometers which overcomes the deficiencies of thosethermometers. The structural change described herein results in aprolongation of the signal retention time often referred to ashysteresis or memory. Concomitantly, this modification also enhances thecontrast between the two states, liquid crystal and focal conic, usefulfor temperature measurement.

The structural change taught here results in a higher contrast signalwhich persists for a time period three to four times longer than thoseachievable with the composition taught in the '317 patent. This isfollowed by a discernible, low contrast signal which persistsindefinitely if it is not mechanically erased. The enhancement of thesignal contrast is achieved also by this structural modification whichallows for greater light gathering property of the liquid crystal. Thiscombination of enhanced contrast and prolonged signal retention allowthe user to make the reading with light of low intensity from eitherincandescent or fluorescent source and even recheck it subsequently ifdesired. A surprising advantage of the inverted structure of thisinvention is that the application of slight pressure to the pressuresensing element results in a reversion to the liquid crystal state ofthose sensing elements which are below the clearing point.

Because the low contrast signal persists indefinitely, a clinicalthermometer incorporating the structural change described in thisdisclosure requires that pressure be applied to the convex domescontaining the thermochromic elements, e.g., by stroking, between usesto clear the previous reading. This is analogous to the shake-downfeature of the clinical mercury-in-glass clinical thermometer, but ismore easily accomplished.

In order to fully appreciate the instant invention it is necessary tounderstand the structure of prior art clinical thermometers preparedutilizing the liquid crystal chemistry of the '317 patent. Thisstructure is described in U.S. Pat. Nos. 4,064,872, and 4,345,470, bothof which are incorporated herein by reference.

The thermometers described in the '872 and '470 patents consist of acarrier layer made from aluminum or a plastic laminate into which aplurality of small spherical or cylindrical pockets or cavities aremechanically embossed. For a clinical thermometer covering the normalbody temperature and fever range in increments of 0.1 C., about 60 suchcavities are required. Furthermore, they must be arranged in a grid-likepattern so that they all experience the same temperature at the site ofmeasurement. Into each of these cavities of about 1 mm diameter and 0.1mm depth is deposited a fixed liquid crystal composition containing thethree compounds cholesteryl chloride, cholesteryl oleyl carbonate andcholesteryl n-butoxyphenyl carbonate and a varying weight per centmineral oil as taught in '317. The sixty or so different compositionsare deposited into the sixty or so cavities simultaneously using amicrodeposition process capable of delivering quantities as small as0.02 milligrams of the specific mixture with an accuracy ofapproximately 15%.

The chemicals are subsequently enclosed in these cavities by sealing atransparent covering film to the non-embossed land area of the carriersurface. These forming, filling and sealing steps result in theproduction of a roll of thermometers which can subsequently be die-cutinto shapes suitable for their intended use.

Referring now to FIG. 1, a cross-section of a cavity containing theliquid crystal composition is shown. The carrier layer, 1, is embossedto form a depression or cavity, 2, into which is deposited the liquidcrystal chemical composition, 3. The embossed cavity or depression, 2,is then sealed with a transparent cover layer, 4, which is heat sealedto the carrier layer, 1, on the unembossed land area, 5. All printing asshown in FIGS. 2 and 3 is done on the underside, 8, of the carrierlayer, 1. Additionally, an overlayer of black pigment, not shown, isapplied to the underside of the carrier layer, 1, in the thermometerfield area (see FIGS. 2 & 3) to make the color of the liquid crystalchemical more visible. In order to protect the black pigment overlayeron the underside, 8, of the carrier layer, 1, from abrasion during themanufacturing process and subsequently during use, an additional layer,7, of either clear or colored plastic film can be adhesively bound tothe carrier layer, 1. Pressure sensitive adhesive coated whitepolypropylene, polyethylene or polyester are satisfactory choices forthis additional layer, 7, application with factors such as cost and easeof die cutting determining the specific choice.

The base or carrier layer, 1, must also satisfy a constellation ofproperties to function in this type of thermometer. The plastic film orlaminate comprising the carrier layer, 1, must be clear so that the backside can be printed black; it must be embossible without cracking to adepth of about 0.2 mm; it must be inert with respect to the liquidcrystals and contain no residual low molecular substances from itsprocessing that could migrate into and contaminate the liquid crystals;it must be heat sealable to other thin transparent inert plastic films.

A preferred material which satisfies these criteria is Kodar® PETGcopolyester 6763 which is a clear amorphous polymer ofpolyethyleneglycol terphthalate ("PETG") with a glass transitiontemperature of 81° C. Kodar is a registered trademark of the EastmanKodak Company.

The upper cover layer film 4, must also be inert with respect to theliquid crystals, transparent, heat sealable to the PETG and contain nolow molecular weight components which can migrate into the liquidcrystal composition through the heat sealing process or subsequentlyduring storage. There are several materials that satisfy these criteriaincluding laminates of PETG 6763, polyvinylidene chloride ("PVDC")coated polyesters such as DuPont's M44 film where the PVDC coating isapplied from an aqueous emulsion and polyester films which are coatedwith amorphous polyester. Because phthalate polyesters do not readilyheat seal to themselves, generally, the cover layer is coated with apolymer which permits heat sealing to these polyester. Preferably theheat sealable component, e.g., PVDC, is deposited from a water emulsion.Solvent deposition will ordinarily result in the retention of sufficientsolvent to adversely effect the performance of the liquid crystalcomposition.

Referring now to FIG. 2 which illustrates a typical centigradethermometer construction in the plan view, the thermometer has a sensinghead section G, with a multiplicity of embossed cavities. Typically, thethermometer has a width, 9, of about 11 mm.

The diameter of the cavities, 10, is about 0.889 mm. The field, 11, canbe about 11.5 mm long, and the rows, 12, and columns, 13, of cavitiesare spaced apart a center to center distance, 14, of about 1.4 mm. Thecentigrade scale, 15, ranging from 35 to 40 degrees Centigrade, runsalong the left end of the sensing head, G. Each row of cavitiesrepresents a degree change in temperature, Along the top of the columnsof cavities are numbers, 16, showing a 0.1 degree change from column tocolumn. The text is about 0.114 mm in height for convenience ofreadability. Extending away from the head, G, is a handle portion, H, bywhich the thermometer can be held when inserting into the mouth or underthe arm, for measurement. The design illustrated in FIG. 2 is in tendedto be held in the right hand for reading. FIG. 2 illustrates athermometer wherein the reading is 37.0° C.

Referring now to FIG. 3 which illustrates a typical thermometerconstruction in the plan view, of a Fahrenheit thermometer, thethermometer has a sensing head section I, with a multiplicity ofembossed cavities. Typically, the thermometer has a width, 17, of about11 mm.

The diameter of the cavities, 18, is about 0.889 mm. The field, 19, canbe about 11.5 mm long, and the rows, 20, and columns, 21, of cavitiesare spaced apart a center to center distance, 22, of about 1.4 mm. TheFahrenheit scale, 23, ranging from 94 to 105 degrees Fahrenheit, runsalong the bottom of the sensing head, H. Each row of cavities representsa degree change in temperature. Along the left and right end of theField, 19, of the columns of cavities are numbers, 24, showing a 0.2degree change from row to row. The text is about 0.114 mm in height forconvenience of readability. Extending away from the head, I, is a handleportion, J, by which the thermometer can be held when inserting into themouth or under the arm, or for reading purposes. The design illustratedin FIG. 3 is intended to be held in either hand for reading. FIG. 3illustrates a thermometer wherein the reading is 98.6° F.

We have found that the liquid crystal compositions described in the '317patent satisfy the criteria for accuracy and stability when subjected totemperatures up to 65° C. Above this temperature the chemistry isirreversibly altered resulting in a loss of accuracy; e.g., at 100° C.the chemicals are adversely effected in approximately twenty seconds ofexposure, Although 100° C. represents an unrealistic temperature fromthe perspective of shipping and storage, it rules out steamsterilization as a process for sterilization before use and puts a limiton heat sealing or other thermal processing conditions.

Thermometers made with the above described materials are both accurateand stable. They are, however, difficult to read for reasons expressedabove. We have discovered a rather surprising phenomenon. Whenthermometers made of this structure are briefly subjected to hot watertemperatures in the range 95°-100° C. the PETG relaxes, collapsing theembossed cavity and gives rise to a structure like that shown in FIG. 4.

Referring now to FIG. 4, the cavity, 2, has virtually disappearedforcing the chemical liquid crystal, 3, upward. The result is that thecover layer, 4, assumes a convex shape. As used in the specification andclaims the term inverted cavity means the convex shape described above.

The exposure time at elevated temperature must be carefully controlledto achieve the desired inversion effect. Prolonged exposure totemperatures in this range results in the failure of the seal betweenthe PETG and PVDC surfaces and destruction of the thermometer. If theexposure time is too short, only partial inversion is achieved and withit no dramatic change.

When the combination of exposure time and temperature is set withincertain limits, this thermal inversion process leads to two surprisingchanges. Firstly, the contrast between the green liquid crystal stateand gray focal conic state of the liquid crystal is enhanced. Notwishing to be bound by theory, it is believed that this enhancement isprobably a consequence of the greater light gathering property of thedome shaped structure of FIG. 4 in comparison to the cavity shape shownin FIG. 1. Secondly, the time before the signal begins to decay isextended from about 20 seconds to sixty seconds when the thermometer isremoved from an environment at 36° C. and placed in an environment at20° C. and from 10 to 30 seconds when the thermometer is removed from anenvironment at 40° C. into a cooler environment. Not only is there thisthree fold increase in memory, but also in the case of the invertedcavity structure it is possible to read the thermometer in a lowcontrast state for many minutes or even hours later. In fact, the liquidcrystals never fully return to their brightly colored liquid crystalstate unless they are mechanically stressed by gentle rubbing.

It is important to recognize that this process of thermal inversion isnot equivalent to viewing the thermometer before it is inverted from thebottom side of the embossed cavities. A thermometer could be constructedto be used in this fashion if the black background printing is appliedto the upper surface of the transparent cover layer; however, themechanical forces on the liquid crystal would be different than thoseproduced from the cavity collapsing and squeezing the liquid crystalbetween the two films.

Since the stability of the liquid crystal compositions is only 20seconds at 100° C., the thermal inversion process must be completedquickly, i.e. less than 20 seconds. Generally, the inversion process iscarried out in about 2 to about 15 seconds; typically, about 4 to 12seconds, e.g., 10 seconds. The time in which the inversion process mustbe carried out is a function of the processing temperature. The higherthe temperature the shorter the allowable time of exposure of the liquidcrystal composition to the processing temperature.

The critical aspect of the inversion process is the temperature to whichthe sensing element is raised. Below 90° C. the inversion does not occurin a reasonable time frame, and for practical purposes is non-existent.Above 120° C. damage to the liquid crystal chemicals is so rapid that itis only with great difficulty that the process can be carried outsafely. Hence the limits of time and temperature for carrying out theinversion process are a temperature range of about 95° C. to about 120°C. over a processing time of about 1 to about 20 seconds. It should benoted that the temperature referred to here is not the temperature ofthe heat source, but the temperature to which the sensing element israised during processing. The air temperature can be about 150° C. toabout 240° C. For example, using a hot air stream at 210° C. andexposure time of 4 seconds results in a successful inversion withoutdamage to the liquid crystal chemical composition because thetemperature of the liquid crystal is not raised above 120° C. for a timeduration which causes damage to the chemicals. While reference to theliquid crystal is made with respect to the inversion processtemperature, it will be appreciated by those skilled in the art havingaccess to this disclosure that the temperature acts on the web to causea mechanical change, e.g., shrinkage, thereby causing the inversion. Itis the restoration of the cavity to substantially its originalunembossed configuration that causes the effect achieved. Generally, theinversion is adequately accomplished for the purpose of this inventionif the embossed cavity shrinks to about 85% of its originally volume.Optimally, the shrinkage results in a decease in volume of the embossedcavity by about 90 to 98%, e.g., 95%. reduction in embossed cavityvolume. As used in the specification and claims the term "substantiallythe same as its original unembossed configuration" as used with respectto the embossed cavity and the inversion process means that the embossedcavity volume has been decreased by at least 90% of its originalembossed volume. Ideally, the embossed cavity disappears entirely andall of the liquid crystal chemical is forced upward to the level of thesurface of the substrate and above to form the domed configuration ofthe inverted cavity.

Four ways to accomplish the inversion process in the requisite timeinterval have been developed.

i. Raising the temperature at heat sealing from the normal sealingtemperature of 130° C. to 180° C. and inverting at sealing. The contacttime on the heated roller is about 0.06 seconds. Because of this shortdwell time, the pressure of 160 bars at the nip rollers and therequirement that the heating must be applied from the PVDC/Polyesterside, only partial inversion of the cavities occurs.

ii. Immersion of the individual thermometers or thermometer roll stockin hot water (90°-100° C.) for a few seconds. This process successfullyinverts the cavities, but distorts the entire thermometer web from whichthermometers are punched into a convex bow and requires that the waterbe blown off all surfaces of the thermometer before the roll stock iswound. Water also permeates into the thermometer affecting the accuracy.Although this absorption is reversible, it delays production andinvolves an additional drying process to restore accuracy.

iii. Bringing the thermometer web into intimate contact with a 2.5×30 cmKapton® insulated etched foil heating element with a power of 60 watts(Omega Engineering) Kapton® is a trademark of Dupont for polyimides.This produces surface temperatures of about 120° C. under dynamicconditions. This also achieves the desired result, but again not withoutprocessing complications and material losses. This approach requiresthat the web be pressed from the upper surface to ensure good contactbetween the bottom surface and the hot platen. This force opposes thedome formation and must be adjusted with great care to insure completeinversion. Contact heating also has the disadvantage that it requiresthat there be either a mechanism for quickly making and breaking contactbetween the heating element and the web when either starting or stoppingor a method to raise and lower the temperature above and below the glasstransition point of the PETG, to prevent melting of the web.

iv. Heating the bottom of the web with hot air through an opening in aninsulated platen just sufficiently wide to allow the hot air to contactthe region containing the cavities. This method minimizes distortion ofthe web and, by controlling the air flow, results in rapid come-up andcool-down times as the process starts and stops. FIG. 5 illustrates theconstruction of a hot air assembly. With a web speed of 5.3 cm/sec, athermometer pitch of 1.2 cm and a heated slot of 19 cm length, thethermal inversion processing time is about 4 seconds at a processing airtemperature of about 210° C. The thermometer pitch is the distancebetween the thermometers on the continuous web of carrier layer.

There is a further constraint on the thermally inverted structure. Theamount of liquid crystal that can be deposited in a cavity must beadjusted such that a small air pocket remains after the thermalinversion. The size of this air pocket must be at least 5% of the volumeavailable for the liquid crystal, e,g., about 10%, and can be as largeas 50% of this space; however, increasing the size of the air pocketreduces the size of the total of liquid crystal and diminishes thereflective area of the signal. In the absence of this air pocket thesignal is of enhanced contrast but persists about only as long as in anuninverted cavity sensing element of the prior art. Not wishing to bebound by theory, it would appear that this air space is needed torelieve the internal pressure that builds in an incompressible fluidwhen the thermometers are cooled; i.e., there may be some slightcontraction of the plastics on cooling which results in an increase inpressure. It has been found that an adequate signal strength is achievedby filling the cavity with liquid crystal chemical to about 50 to about90% of its capacity. Optimal results are achieved when the cavity isfilled with about 65 to 85% of liquid crystal the balance being air.Typically about 70 to 80%, e.g., 75% of the cavity is filled with liquidcrystal composition. This results in approximately a 10% to 50% airfilled void upon cooling, e.g. about 10 to about 30% where the liquidcrystal comprises about 90 to about 70% of the cavity volume.

in addition to resulting in a prolonged signal, it has been found thatthe dome shaped structure of the inverted cavities allow for easyerasure of the signal. It is necessary only to apply pressure to thesensing elements to erase the signal, e.g., by wiping the raised, domedsurfaces gently with a finger or cloth to clear the old signal.

The inverted structure of this invention results in a clinicalthermometer which satisfies the criteria set forth above with respect torange, accuracy, resolution, stability, and ease of reading and clearingthe signal .

Although the thermometer of this invention can be used repeatedly, in ahospital environment it may be of greater acceptability as a single usedisposable. In this application the extended memory would require that aprevious signal retained on the thermometer produced by elevatedtemperature during storage and distribution be cleared prior to use.This can be accomplished quite easily with a dispenser which rubs thedomes lightly when a unit is withdrawn. Another way a residual signalcan be cleared before use is by wrapping thermometer between two layersof cohesively coated film or paper. When the paper is peeled open andthe thermometer withdrawn, the gentle contact is sufficient to clear thethermometer.

A preferred embodiment of this invention is as follows:

The preferred substrate is Kodar PETG copolyester of 0.20 mm thickness.This material is printed on one side with the black background thatabsorbs the components of light not reflected by the liquid crystals. Anadditional layer of white polypropylene of 0.10 thickness is adhesivelybonded to the printed side of the PETG. This polypropylene layer servesthree roles. It protects the printed surface from abrasion during theweb transport process, it stabilizes the PETG against distortion whenthe pockets are thermally inverted, and it increases the opacity of theliquid crystal background. The printed laminate is first embossed with60 cylindrical shaped cavities of 1.0 mm diameter and 0.10 mm depth.Next, approximately 30 micrograms of 60 different compositions of liquidcrystal following the teaching of '317 are deposited into each of thesecavities. Following the fill step a covering film of DuPont's M44, PVDCcoated polyester, 0.013 mm in thickness is heat sealed to the PETGacross the land area between the embossed regions encapsulating theliquid crystal sensors. These filled and sealed cavities are theninverted by directing a stream of hot air against the web'spolypropylene surface through a 2.5×19 cm opening in an insulatedplaten. The air temperature at the web surface peaks at 210° C. and theweb temperature is about 95° to about 120° C. The thermometer web isthen cooled by blowing room temperature air over the web, and wound intoa roll of about 300 meter length. Individual thermometers with shapeslike those shown in FIGS. 2 and 3 are subsequently cut from the rollusing a steel die set.

The fill step is carried out with any suitable micro-depositionequipment. U.S. Pat. No. 3,810,779, discloses a typical fill system.This fill system operates on gravity feed. In view of the high viscosityof the liquid crystal chemicals, a modification of the '779 system whichapplies a pressure to the chemical storage reservoirs is preferred. apressure of 1 to 3 PSI (about 50 to 150 tor) is adequate. The moreaccurately pressure is controlled, the less effected is the depositionprocess by level of the chemical in the storage reservoir. In oneembodiment the reservoirs comprise hypodermic syringes from which theplungers have been removed, and to which an air source at controlledpressure is applied. It will be appreciated by those skilled in the artthat a pressure regulator of good quality, e.g., sensitivity of +5 tor,and a drift of less than 5 tor, must be utilized to maintain thepressure applied to the syringe bodies within a predetermined pressurerange. Where the chemical is a liquid crystal chemical composition,heating is neither required or preferred, since prolonged exposure toelevated temperatures will degrade the chemicals.

Referring now to FIG. 5A, a typical equipment arrangement for carryingout the inversion process is shown. A centrifugal fan, 25, supplies airat a velocity of about 47 CFM to a heating zone, 26, comprising heatingelements of about 1500 to about 2000 watts of output. The heated air,not shown, at a temperature of about 210° C. passes through a duct, 27,into a heat shield, 28, which directs the air into the inversion zone,29, against the product web, 30. It should be noted that it is not theair temperature which is critical, but the temperature to which the webis raised to cause the inversion. The heaters, not shown, extend thelength of the duct, 27, into the heat shield, 28. As shown in thedrawing the product web, 30, is moving in a direction into the paper;that is, the view shown is a cross section of the web across the machinedirection of the web. The web, 30 is supported by a teflon insulator,31, which is supported on an aluminum platen, 32. Both the insulator,31, and the platen, 32, have a slot, 33, there through in the inversionzone, 29, thereby giving the heated air access to the web 30. Since itis essential not to overheat the web, 30, the platen, 32, is cooled bycooling fins, 34. Additionally, a cooling fan, 35, shown partially cutaway to expose the heat shield, blows ambient air over the lower surfaceof the platen, 32, to insure that the web, 30, and in particular theliquid crystal chemicals, 36, of the thermometers are cooled rapidly andare not heated above about 95° to about 110° C.

The platen, 32, and centrifugal fan, 25 are supported on supportstructure, 41, The centrifugal fan, 25, together with the duct, 27, andheat shield, 28, rest on a support plate, 37, which is rigidly mountedon a movable plate, 38. The movable plate, 38, which is secured bybolts, 39, to the base, 40, of the support structure, 41, can move in avertical direction in guide slots, 42, when the bolts, 39, are loosened.This permits the entire fan and heat shield assembly to be moved awayfrom the web, 30, for servicing. Additionally, it permits location ofthe heat shield, 28, to be adjusted in close proximity to the web, 30.

FIG. 5B is a cross section of the equipment arrangement along sectionA--A as shown in FIG. 5A, showing the upper portion of the duct, 27. Theweb, 30, moves in a machine direction from left to right, and issupported on the insulator, 31, and platen, 32. Flow lines, 43, show,schematically, the heated air flow out of the duct, 27, into the heatshield, 28, across the inversion zone, 29, and downward exiting throughthe lower end, 28a, of the heat shield, 28.

The cooling fan, 35, blows ambient air along the bottom of platen, 32,and along the cooling fins, 34, to cool the platen and web, 30,downstream of the heat shield, 28.

The thermometer of this invention with its inverted cavity meets the 20second signal retention requirement of ASTM E1299-89 for a reusablethermometer as well as the one minute signal retention requirement ofASTM E825-92 for a single use thermometer.

When used as a clinical thermometer to measure temperatures orally oraxillarily, the liquid crystal compositions of the '317 patent aresubjected to temperatures of about 35° to about 41° C. When removed fromthe site of the heat source (under the tongue or arm) the thermometer isbrought into an ambient temperature of about 20° C. This thermal shockis enhanced by evaporative cooling increasing the temperature differenceby as much as 5° C. Under those circumstances there is a rapid reversionfrom the focal conic state to the liquid crystal state which takes placein about 20 seconds. Utilizing the inverted cavity or pocket of thisinvention, that reversion time is extended to as much as several hoursunder the same conditions. When, however, the temperature to which thesensing element is exposed varies about 4° C. or less, the signalretention time is in the order of several days.

This phenomenon can be utilized to continuously monitor bodytemperature. Taking 37° as the "normal" body temperature, there israrely a 4° C. swing in body temperature even where fever is involved.Hence, when the thermometer of this invention is applied to anappropriate body site for measuring temperature, it will always read thelast maximum body temperature since no reversion will occur betweenreadings. On the other hand after reading the maximum temperature, theapplication of pressure, e.g., by rubbing the domed surfaces of theinverted cavities, results in reversion to the liquid crystal state ofthose sensing elements of the thermometer whose liquid crystalcompositions are trapped in the focal conic state and whose transitiontemperature is above the body temperature at the measurement site, andimmediately thereafter a change to the focal conic state for thetemperature at the body site, thereby displaying the present bodytemperature. A typical construction of a thermometer which can be usedto continuously read body temperature is shown in FIGS. 6A and 6B.

FIG. 6A depicts the plan view of a typical thermometer for continuousbody temperature monitoring. The thermometer field, 44, is laid out intwo sets of columns. The first set, 45a, covers the temperature range of96.0° F. to 99.8° F. The second set, 45b, covers the temperature rangefrom 100° to 104.8° F. While this arrangement differs from that of thethermometer of FIG. 3, the difference is one of choice, and not relevantto the function of the thermometer. Unlike an oral thermometer whereplacement is critical, and therefore, the arrangement of the sensingelements may affect accuracy, here no such criticality exists. Thethermometer has a pull tab, 46, the function of which is readilyapparent from FIG. 6B. Referring now to FIG. 6B, the thermometer, 7, hasan adhesive layer, 48, which extends the length of the thermometer up tothe pull tab, 46 which is free of adhesive. The adhesive layer iscovered with a release paper, 49. The thermometer is applied to the bodyby striping the release paper off of the thermometer and pressing theadhesive side to the body. The sensing elements, 50, of the thermometerare the inverted cavities of this invention. When the thermometer, 47,is adhered to the body, the pull tab, 48, is unattached. It can be usedto release the thermometer from the body when it is to be discarded.

The thermometer can be used in the axillary position. However, it ismost advantageously used on new born infants by applying to the abdomenin the area over the liver. This body site is alleged to approximatecore temperature. A nurse attending the infant can read the maximumtemperature of the infant since the last visit by the nurse. The nursecan then apply pressure to the thermometer, thereby erasing so much ofthe signal which is represented by sensing elements whose transitiontemperature is above the body temperature at the thermometer location.After about 5 seconds the current temperature of the infant can be read.

While the concept of inverting the cavity containing the liquid crystalcomposition when applied to the thermometer of this invention requiresthe entrapment of air to cause the prolonged signal retention, it can beapplied to prior art liquid crystal thermometers to enhance readability.In the prior art thermometers it is desirable to eliminate the toroidalconfiguration of the liquid crystal composition formed when the liquidcrystal composition is deposited in the cavity. A coin shapedconfiguration of the liquid crystal composition which appears to fillthe cavity is preferred from the standpoint of readability. This isaccomplished by displacing air in the cavity with helium prior to heatsealing the transparent cover to the embossed substrate. The structureof this filled cavity is essentially as shown in FIG. 1. After heatsealing the helium escapes from the enclosed cavity at a rate whichexceeds the ability of air to reenter the pocket by diffusion. As aconsequence a reduced pressure is formed in the cavity, resulting in thecover film being pressed down against the liquid crystal chemical by thedifference in pressure across the cover film. The result is that thetoroid disappears, and the chemical appears to fill the cavitycompletely in a coin shaped configuration. The readability of this priorart thermometer is, however, dependent on the angle of light strikingthe thermometer, and users must be trained to read the thermometer byadjusting its orientation to available light.

in general the procedure for preparing a thermometer are as follows: Aprepared web comprising PETG having the thermometer scale printed on theunderside, and overlayed with a protective polypropylene film is fedinto an embossing station of a thermometer production line. Afterembossing the web moves to a fill station where liquid crystalcomposition is deposited into the embossed cavities. Thereafter, the webis fed into a heat sealing roller nip where it is merged with a coverlayer which is heat sealed to the web. The thermometer web leaves thenip with the cover layer adhered to the web in the area of the field,thereby sealing the liquid crystal composition in the cavities. Thesealed thermometer structure is then cooled and subsequently die cut tothe desired shape. For a line speed of about 250 cm/sec, the heatedroller surface temperature is about 134° C. to effectuate an adequateseal.

If the cavity inversion process is applied to the prior art, heliumtreated thermometer the result is that the thermometer is less dependenton the angle of light striking the thermometer than in the case ofthermometers with uninverted cavities. Additionally, the domeconfiguration causes the cavity to appear larger and the thermometer ismore easily read by an untrained user.

Although helium is lighter than air it has surprisingly been found thathelium can be introduced into a thermometer cavity by directing a streamof helium into the nip of rollers utilized to heat seal the cover layerto the substrate. The conventional method of sealing the cover layer tothe substrate is to pass the embossed substrate with its chemical filledcavities and the cover layer into the nip of a pair of rollers one ofwhich is heated.

Referring now to FIG. 7 an embossed web, 51, containing cavities, notshown, to which liquid crystal compositions, not shown, have been addedis passed through the nip, 52, formed by heated roller, 53, and apressure roller, 54. The thermometers are aligned on the web so thattheir longer dimension is parallel to the axis of rotation of therollers. The cover layer, 55, which 1s a laminate of polyester and PVDCpasses over a guide roller, 56, around a section of the heated roller,51, and into the nip, 52, where the web, 51, comprising PETG 1s sealedto the cover layer, 55, by melting of the PVDC layer of the cover layer,55. The cover layer, 55, enters the nip, 52, with the PVDC side of thecover layer laminate, 55, in juxtaposition with the PETG layer of theweb, 51. Since the heat sealing temperature is about 134° C., thepolyester layer of the cover layer laminate is not melted by the heatedroller. 53. The sealed web, 61, then passes through cooling rollers, notshown, which cool the thermometers prior to winding into a roll fortransfer to a die punching station, not shown, for punching out theindividual thermometers from the web. The cooling rollers are driven,and pull the web, 51, and cover layer, 55, through the nip, 52. thepressure applied by the pressure roller should not be so great as tocrush the cavity. Generally an air pressure of about 40 pounds appliedto pistons used to urge the pressure roller against the heater roller isadequate.

Where helium is to be introduced into the cavities a stream of helium,65, from a helium line source, 57, is fed into a nozzle, 58, whichdirects the helium stream, 65, directly into the nip, 52, of therollers. In the process of doing so the helium flushes the air out ofthe cavity, not shown, and fills the cavity. Diffusion, of the heliumout of the cavity takes place over about twenty to thirty minutes. Theinversion of the cavities is carried out in the manner described abovebefore die punching the individual thermometers.

As shown in FIG. 8, which is an end view of the thermometer sealing lineshown in FIG. 7, the pressure roller, 54, is slightly wider than thesealing section, 63, of the heater roller, 53. The sealing section, 63,of heated roller, 53, is raised so as to seal the cover layer 55, onlyto the area about the field, 19, of the thermometer, 64. The sealingsection, 63, of the heated roller, 53, is the width of the cover layerwhich is to be sealed to the web, 51, at the field, 19, only. The heatedroller, 53, is a chrome plated polished roller while the pressureroller, 54, is encased with silicone rubber. The guide roller, 56, has arecessed section, 60, in which the cover layer, 55, rides, and an outerlarger diameter, 61, which guides the edge of film. The thermometeroutlines shown in FIG. 8 are for illustration purposes only. They do notexist in practice. The shape of the thermometer will depend on thedesign of the die used to punch out the thermometers from the web. Priorto die punching only the thermometer field is visible on the web.

What is claimed is:
 1. A temperature sensing element comprising a voidvolume, said void volume being defined by a substrate and a cover layer,the void volume containing therein a composition comprising at least onecholesteric liquid crystal chemical temperature sensing compositionwhich occupies about 50 to about 90% of the volume of the void volume,the composition being configured in a toroidal shaped ring along acircumferential region defined by the intersection of the substrate andcover layer, the cover layer having a convex configuration; the liquidcrystal chemical temperature sensing composition being in contact withthe convex cover layer and having a convex shape, that portion of thevoid volume not containing the liquid crystal composition containingair.
 2. The sensing element according to claim 1 wherein the cholestericliquid crystal composition comprises about 57.9 weight %, based on theliquid crystal composition, of cholesteryl oleyl carbonate; 30.7 wt. %,based on the liquid crystal composition, of cholesteryl chloride, and11.4 wt. % cholesteryl-n-butoxyphenyl carbonate, based on the liquidcrystal composition, diluted with a mineral oil.
 3. A thermometercomprising a multiplicity of sensing elements according to claim
 1. 4.The thermometer according to claim 3 wherein the cholesteric liquidcrystal composition of the sensing elements comprises about 57.9 weight%, based on the liquid crystal composition, of cholesteryl oleylcarbonate, 30.7 wt. %, based on the liquid crystal composition,cholesteryl chloride and 11.4 wt. % cholesteryl-n-butoxyphenylcarbonate, based on the liquid crystal composition, diluted with amineral oil, each of said cavities containing a different amount ofmineral oil than each of the other cavities, thereby defining atemperature range over which the thermometer is operative.
 5. Thesensing element according to claim 1 wherein the substrate comprisesPETG and the cover layer comprises polyester coated PVDC.
 6. Thethermometer according to claim 3 wherein the substrate comprises PETGand the cover layer comprises polyester coated PVDC.
 7. The thermometeraccording to claim 4 wherein the substrate comprises PETG and the coverlayer comprises polyester coated PVDC.
 8. A method for continuouslymeasuring a body temperature of a patient which comprises adhesivelyapplying the thermometer according to claim 4 to a body area of thepatient which has a temperature with a fixed relationship to a coretemperature of the patient; reading the thermometer to ascertain amaximum temperature attained by the patient since a prior reading of thethermometer; applying pressure to the thermometer to cause sensingelements of the thermometer which have a transition temperature which isbelow the temperature of the body area to revert to a liquid crystalstate, waiting for a time sufficient for the thermometer liquid crystalcomposition to change to a focal conic state consistent with thetemperature of the patient, and reading the patients temperature.
 9. Thesensing element according to claim 1 wherein the liquid crystalcomposition comprises about 65 to about 85% of the void volume.
 10. Thesensing element according to claim 1 wherein the liquid crystalcomposition comprises about 70 to about 80% of the void volume.